JX-8P repair

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LektroiD
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JX-8P repair

Post by LektroiD » Wed Dec 18, 2013 10:23 am

I have a JX-8P which had a few faults that I've cured, but the synth has developed more in the process. Hopefully someone here might be able to shed some light.

- I stripped the keyboard down as there were hanging notes and the aftertouch didn't work.
- I also took out the main PCBs to repair the display which wouldn't light up.

Now I have a problem where none of the input devices work:
- keyboard almost dead, occasional key (usually the same key each time) plays a cacophony of all 6 voices at once.
- None of the sliders work, volume for example is fixed output regardless of where it is set.

I don't know if I've dislodged something I shouldn't have, or if I've broken the keyboard in my attempt at repair. I have had it out twice to make sure continuity is good at key on & off, I also tested the cables to the motherboard for continuity, everything seems to check out fine.

There was a problem with the data slider, but I can't see that causing all input devices to suddenly stop working (keyboard, volume, pitch bend, etc).

It plays through MIDI fine.

I can't think of anything else, hopefully someone here might be able to shed some light on it.
Last edited by LektroiD on Wed Dec 18, 2013 2:03 pm, edited 1 time in total.

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raisinbag
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Post by raisinbag » Wed Dec 18, 2013 12:54 pm

I had a massive fail recently. Power supply went fuckidy so I'm waiting on some voltage regulators in the mail an hope that helps. I started to have probs with mine and then suddenly NOTHING! Checked power and it was way off. So maybe check your supply and see if anything if off voltage wise. Just a thought. For the other stuff I'm useless right now but once I get power working again I may be having a crash course in jx8p inner workings so we can scratch out heads together then.

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Post by Thonk Support » Wed Dec 18, 2013 1:09 pm

My JX-8P case is really knackered... I had considered detaching the keyboard completely and putting it in a tabletop box... so interested to hear the thread progress.
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LektroiD
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Post by LektroiD » Wed Dec 18, 2013 2:12 pm

The power supply checks out fine, I thought as much since it all works ok through MIDI (although power starvation can affect things in strange ways).

I'm desperate to get this synth working. It just seems odd that it was just a couple of hanging notes, then after reassembly none of the input devices work (although the tact switch panel is working as normal).

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Post by rekem1000 » Wed Dec 18, 2013 3:42 pm

If it plays though midi fine then it could be a bad ic somewhere on the data bus between the gate array and cpu or possibly the gate array itself which is not good.
The blank screen is most likely due to a bad display driver coil, small grey cylinder like transformer, it's a common problem with all JX's and they are next to impossible to find unfortunately.

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Post by LektroiD » Wed Dec 18, 2013 3:53 pm

rekem1000 wrote:If it plays though midi fine then it could be a bad ic somewhere on the data bus between the gate array and cpu or possibly the gate array itself which is not good.
The blank screen is most likely due to a bad display driver coil, small grey cylinder like transformer, it's a common problem with all JX's and they are next to impossible to find unfortunately.
I sorted the display (sorry I wasn't clearer in the opening post), I just retouched the solder joints and that now works fine. Getting to it required me stripping the motherboard and switch board out, and unplugging most of the loom. It was when I put it all back together when the problems with the keyboard, volume, pitch bend, data slider, etc. started. I'm wondering if it's something I may have dislodged in the process. I've never known a chip to go bad from moving a board.

If that is the case, how would I go about testing it?

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LektroiD
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Post by LektroiD » Wed Dec 18, 2013 6:12 pm

On further inspection of the service manual, it appears the gate array is only relative to the keyboard. Whatever has happened here has knocked out the entire volume board and pitch bender, as well as the keyboard... I'm guessing these are not isolated problems, since it all occurred when I reassembled the synth.

I'm going to strip it down again, have a close look at all the connections, etc. do a continuity check throughout (I'm wondering if it may be a bad ground in that section). Hopefully reassembling it may bring it back. I'll report back as soon as I'm done.

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Post by Dr. Sketch-n-Etch » Wed Dec 18, 2013 8:37 pm

Man, LektroiD, your JX-8P sounds exactly like mine. The keyboard is totally non-functional. I haven't bothered to try to fix it yet -- wondering if it's even worth it...
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Post by LektroiD » Thu Dec 19, 2013 6:46 am

Update:

I thought it may be an idea to retouch the data bus chip, so I put in my ultra thin Weller tip, grabbed my super thin solder wire, put some liquid flux all round and retouched the SMD (I really need to invest in a rework station).

When I put it back together, it wouldn't work at all (DOA except one LED lit), so again, I took the motherboard to the workbench and retouched the chip once again, checking it with my bench magnifying light. There must have been some residual flux under the pins as it hissed and bubbled on a couple of pins.

When I went to put the board back in, I noticed I had been putting the 8 pin cable from the volume board into the 8 pin keyboard socket, and vice-versa. How stupid do I feel now :doh:

Needless to say, I have a fully working JX-8P. The keyboard and aftertouch is really responsive, and despite feeling rather silly, I am delighted to have the board up and running again.

I would like to source the 4x 30mm 10KB sliders for the volume board though.

Thanks for all the help and support with this, despite it being my own stupidity. :eek:

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Post by raisinbag » Tue Dec 24, 2013 11:38 am

Woo hoo glad you got your working. I'm gona hijack your thread. I finally fixed my power supply issue. It is now totally recapped, had a transistor replaced and the final component swap which did the trick was the funky voltage regulator. So I now have correct voltages for the regulated voltages though the unregulated 7v is closer to 10.5. But someone mentioned that this isn't so important. Anyway when I flicked the on switch I had NO SMOKE! synth booted up and sounds in backs diss play as if storage and memory are working fine.
Here is issue: when I play the sounds they are not correct. For example, piano sounds are a hollow noise based sound with a faint piano like pitched/attach but very low volume. Mixed with that is a fluctuating filtered noise source. Other patches are much better like the pads have a fuller sound but I think they are missing something. I have a repair doc somewhere. Any suggestions where to start? As a total mr green to something this complicated my gut says one or more of those "voice" sections got effected by the over voltage from the power supply going bad. Any suggestions regarding where to begin my investigations and possible a methodology of trouble shooting orders would be amazing.

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Post by LektroiD » Tue Dec 24, 2013 12:40 pm

I wouldn't consider your post a hijack, the intent behind the opening post is for people to share knowledge about the JX 8P, and hopefully help others in the future.

When I get back to the studio, I'll do a bulk dump and upload the file here to see if that corrects the issues. This is really the only way, as it doesn't have a factory reset, so hopefully the bulk dump will reinitialise the presets too (?).

In the meantime, go through the various parameters switching on/off various functions, such as cross modulation, sync, chorus modes etc. to see if you can recreate (or cure) the hollow sound. That way you can trace which part of the circuit is causing the problem.

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Post by raisinbag » Tue Dec 24, 2013 12:49 pm

Haha I was just being my silly smart ass self re hijack comment

Well I am tearing down this giant ass 16' high tile wall so if no tiles fall and mame me, I'll go home put kids to bed, dawn my Santa suit and then trade the Santa suit for my geeky synth hat with a propeller o top and follow your lead. As well as try to reset with your bulk dump. Thanks man

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Post by raisinbag » Mon Jan 06, 2014 9:13 pm

Hey LektroiD, so wow, I never really had delved so deep into this synth. I had honestly picked it up crazy ass cheap at one point and just used the factory sounds. I grabbed a $5 app for my kids Ipad and used that to scroll through functions as well as did it manually, in order to test out each parameter. OMG when I get this working properly again I'm gona have some fun, kinda needed a kick in the ass to delve into it.

Anyway these seem to be the issues I am encountering, maybe there are more, and maybe some are not as bad as they seem, it is hard to know since I do not have GURU status with this machine.

VCO1 seems to be "FUCKED UP" I seem to get no wave forms from VCO1. Only situation I am getting sound out of VCO1 is the Noise wave from. Ramp, Pulse and Sqr are giving me no output. BUT yes noise does.

VCO2 Does give me sounds for each wave form, so thats good. The Xmod is doing wierd stuff, but im not sure if that is what it normally does, or is realted to VCO1 Being F'ed up.

Other bad news seems that the Chorus seems to be dead as well. When I change the values of the Chorus parameter I get no audible change.

There are two other strange artifacts going on. 1) there seems to be a constant very low but audible noise source which is fluctuating in volume either due to it being "filtered" or "attenuated". If is very slight, but I doubt this is supposed to be going on. 2) there seems to be a note ringing in the background. Very low in volume, but it seems to be related to a last note I play. But then will ring and not be affected by any of the new notes im playing. Like a sustain going on forever.

Well........... This is out of my experience so far, so I am a bit scared and a bit excited. I have learned here that noting is impossible to fix, but can take time, patience and knowledge, so I really look froward to going through the paces.


Cheers, Mr Baggins

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Post by raisinbag » Wed Jan 08, 2014 10:33 pm

So kinda bumping my last post so I might get some help beginning to solve my hurtin unit.

So for the Chorus, if it isn't working what is the most likely issue? I'm guessing that the 3009 got fried? or would it be the 3101 that clocks it that might have got messed. What is best way to test, other than buying new chips?


Ok, for the DCO1 giving me no wave form, I'm assuming again that something got fried. IS it possible that every 4066 in all voices got zapped? Or is it more likely that a chip before or after is the issue? I'm not really sure where to begin, so was hoping for a poke in the right direction.

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Post by zaphod betamax » Wed Oct 14, 2015 12:34 pm

I had one DCO totally disappear. Check ALL the cabling harnesses!
That fixed mine.

raisinbag wrote:So kinda bumping my last post so I might get some help beginning to solve my hurtin unit.

So for the Chorus, if it isn't working what is the most likely issue? I'm guessing that the 3009 got fried? or would it be the 3101 that clocks it that might have got messed. What is best way to test, other than buying new chips?


Ok, for the DCO1 giving me no wave form, I'm assuming again that something got fried. IS it possible that every 4066 in all voices got zapped? Or is it more likely that a chip before or after is the issue? I'm not really sure where to begin, so was hoping for a poke in the right direction.

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Re: JX-8P repair

Post by Kelvisen » Sun Nov 22, 2020 1:46 am

Hello! I have picked up a beat up JX-8p that has a couple of demons. The screen is very intermittent, some times it works, sometimes it displays gibberish, sometimes nothing. Sometimes turning the synth off and on brings the screen back and some times hitting buttons makes it display the right or wrong thing. It seems like its doing the right thing when buttons are hit just not displaying correctly. The second issue is one key doesnt work 95% of the time. very rarely it will trigger. Advice on what to investigate appreciated!

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Re: JX-8P repair

Post by bitflip » Sun Nov 22, 2020 7:50 pm

Kelvisen wrote:
Sun Nov 22, 2020 1:46 am
Hello! I have picked up a beat up JX-8p that has a couple of demons. The screen is very intermittent, some times it works, sometimes it displays gibberish, sometimes nothing. Sometimes turning the synth off and on brings the screen back and some times hitting buttons makes it display the right or wrong thing. It seems like its doing the right thing when buttons are hit just not displaying correctly. The second issue is one key doesnt work 95% of the time. very rarely it will trigger. Advice on what to investigate appreciated!
Sounds like sometimes-bad connections somewheres. Open 'er up, and if nothing is obvious on visual inspection, CAREFULLY unseat and reseat all connectors and ICs in their sockets. And you can check the battery and PS voltages, but hopefully just re-establishing a bad contact across a connector will fix it.

Intermittent key problems are often from dirty contacts, fixed by disassembly, cleaning, and reassembly - tedious work, but not too difficult - a clean workspace and a little wine help ;)

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Re: JX-8P repair

Post by gruebleengourd » Sun Nov 22, 2020 11:53 pm

Kelvisen wrote:
Sun Nov 22, 2020 1:46 am
Hello! I have picked up a beat up JX-8p that has a couple of demons. The screen is very intermittent, some times it works, sometimes it displays gibberish, sometimes nothing. Sometimes turning the synth off and on brings the screen back and some times hitting buttons makes it display the right or wrong thing. It seems like its doing the right thing when buttons are hit just not displaying correctly. The second issue is one key doesnt work 95% of the time. very rarely it will trigger. Advice on what to investigate appreciated!
Reflow the solder connections on the psu. Especially anything that looks crusty.

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